1. Field of the Invention:
This invention relates to compositions of matter comprising a saturated polyester and a cross-linking agent therefor and, more especially, relates to such resinous compositions further including a minor amount of a certain metal oxide and to the use of such compositions as binders for powder paints.
2. Description of the Prior Art:
Compositions of matter useful as binders for various of the powder paints and comprising a cross-linkable saturated polyester resin bearing terminal hydroxyl groups and an anhydride curing agent therefor are known to this art. Compare the U.S. Patent No. 4,024,111 to Thomas et al. These and other known compositions comprising the powdered paints, moreover, typically include a given charge of pigment, with the ratio of the charge to binder varying over wide limits. Also typically included are such additives as spreading or flow control agents [which modify the surface tension of the paint], catalysts, colorants and the like. Preparation of the aforesaid compositions has been readily effected, usually by separately formulating the binder resin and the curing agent, optionally involving admixing and fusing the same together at a temperature lower than that at which curing or interreaction occurs, by next admixing the product of such fusion of the binder components with the pigment charge and any other additives or adjuvants at low, non-curing temperatures, followed by the ultimate grinding or powdering of the resulting mix to any suitable granulometry. The ultimate composition is thence coated onto any suitable substrate, typically a metallic substrate, e.g., by electrostatic deposition or spraying, and, subsequently, the substrate thus coated with such a powder is placed into any suitable oven or the like at a temperature in excess of that required to cause fusion and spreading of the powdery composition and, concomitantly, the cross-linking or curing of the binder. In this manner, a smooth and glossy coating is obtained. It too is known that the ultimate mechanical properties of a coating thus provided are functions of the degree or amount of crosslinking and, accordingly, of the curing time. Hence, to obtain the optimum in mechanical properties, to date it has been required that the known powdered paint compositions be cured over prohibitively long periods of time.
Thus, a great need exists in this art for improved, paint compositions which permit of the attainment of paint coatings having the most desirable mechanical properties, but within the more acceptable periods of time demanded by truly economical industrial processing.